Clement Greenberg, 1960: “The first mark made on a canvas destroys its literal and utter flatness, and the result of the marks made on it by an artist like Mondrian is still a kind of illusion that suggests a kind of third dimension. Only now it is a strictly pictorial, strictly optical third dimension.” Here Greenberg fastidiously refined his earlier pronouncements on painting’s intrinsic qualities. The picture plane’s “heightened sensitivity,” he explained, was such that even the taut weaves of Mondrian’s last compositions still induced the pleasant vertigo of illusory depthalbeit a “strictly optical” depth that lacked the navigable coordinates of linear perspective. In 1968, Leo Steinberg mocked this notion of optical space as a NASA-era fantasy of zero-gravity spectatorship. (Morris Louis, meet Major Tom.) By contrast, James Hoff suggests that the picture plane’s “